The present invention relates generally to radar receivers, and more particularly to a method and apparatus for distinguishing between signals received from a desired target and those received from unwanted clutter.
It is well known that radars operate by transmitting a signal at a radio frequency (hereinafter referred to as the transmitted signal). The transmitted signal is reflected by a target and returned as a received signal. A received signal returned by a target moving radially with respect to the radar transmitter has a frequency different from that of the transmitted signal. The difference between the frequency of the transmitted signal and the received signal, known as the Doppler shift frequency, may be used to determine the range rate between the target and the transmitter. If several targets having different range rates are illuminated by the transmitted signal, the resulting received signals returned by each target are distributed within a band of Doppler shift frequencies. The location of the received signal returned by each target within the Doppler frequency band is thus a function of the range rate of the associated target. If the range rate of a particular desired target is greater (or less) than the range rate of certain other undesired targets, filtering techniques may be used to separate the received signal returned from the particular desired target from the received signals returned from undesired targets.
A problem occurs when the Doppler frequency of a desired target is close to the undesired target's Doppler frequency. This is particularly troublesome when the undesired targets include ground clutter, as received signals returned from ground clutter are often much more powerful than received signals returned from desired targets. In radars using a transmitted signal of the continuous wave (CW) type, one way to improve operation has been to use a bandpass filter with a passband including the expected Doppler frequency of a desired target and stopband which includes the expected Doppler frequency of the clutter. Sometimes both conditions cannot be met simultaneously. This may occur when detectability of hovering or low radial velocity objects such as helicopters is important. If digitally implemented, such bandpass filters often require the use of weighting having extremely low side lobes, such as the Blackman-Harris weighting described in Harris, F., "On the Use of Windows For Harmonic Analysis With a Discrete Fourier Transform", in Proceedings of the IEEE, vol. 66, No. 1, January 1978. The use of such weighting reduces sensitivity because the main lobe of the filter's frequency response becomes wider, and because a 3 db processing loss is introduced.
For radars using a pulsed transmitted signal, an additional disadvantage exists if the bandpass filter technique is used. Because such a filter normally has a relatively narrow bandwidth to effect the desired discrimination between desired and undesired signals, its temporal response will be longer than the duration of any return signal. This in turn has a negative effect on range resolution and detectability of targets at short range. One prior solution to this problem has been to use range gates to quantize the return signal into range intervals. U.S. Pat. No. 3,706,095 to Cherwek is an example of such a system where the return signal is first range gated before being fed to a bandpass filter. If range gates are used, they normally must be applied after detecting the pulsed transmitted signal, because clutter created at the harmonics of the code or pulse repetition interval will not otherwise be removed.
It is also known that improved performance can be achieved by using an adaptive filter to center the clutter stopband at the average clutter Doppler frequency within selected range cells. See for example U.S. Pat. No. 3,742,500 to N. Freedman and U.S. Pat. No. 3,987,442 to McLeod, Jr. The use of such adaptive filters requires an attendant increase in design complexity and cost.